1. Field of the Invention
This invention relates to a magnetic field-generating device which employs a permanent magnet and which can generate a powerful, accurate and uniform magnetic field in a large air gap, and more particularly to a magnetic field-generating device which can be used for nuclear magnetic resonance-computerized tomography (hereinafter referred to as NMR-CT) so as to obtain a sectional image of an object subjected to medical examination and picture the properties of tissue.
2. Description of the Prior Art
In order to obtain a desired tomographic image of a human body by placing all or part of the body in an air gap where a strong magnetic field on the order of 1-10 KG (0.1-1 T) is created, a uniformity and stability on the order of 10.sup.-4 or less are required. As the magnetic field generating device for NMR-CT, a device is known which includes a resistive electromagnet composed of cylindrically wound copper or aluminum coils. Another known device includes a superconducting magnet which is cooled to near the temperature of liquid Helium and is composed of special conductor coils.
The former device, due to its structure, is low-priced; however, in order to generate a sufficiently strong magnetic field, it consumes huge amounts of electric power and cooling water. As such, its operating cost is high, and its available field strength is practically limited to around 0.15-0.20 T. On the other hand, the superconducting magnet of the latter device has the advantage that it can generate a stronger magnetic field with little power consumption, but its price is much higher than that of the usual conductor; furthermore, the use of costly liquid Helium, is indispensable to achieving a super conducting condition. As such, the total cost of the device is too expensive to be widely used for medical applications.
In comparison with the foregoing, a magnetic device field-generating device employing a permanent magnet was suggested by OMR Co. and FONAR Co. in the RSNA in December, 1982. This suggested device, however, weighs about 100 tons, which is too heavy for usual hospital usage.
There has also been proposed an alternative device wherein a pair of circular ring-like permanent magnets which are magnetized in the axial direction and which are spaced apart and oriented such that the facing sides have opposite polarities (see JA-OS No. 61763/59, laid open for public inspection on Apr. 9, 1984), and a device wherein plural trapezoidal permanent magnets are arranged annularly (see JA-OS No. 501995/59, laid open for public inspection on Nov. 29, 1984). The former device is desirable as far as costs are concerned, but its leakage flux is large because it creates a magnetic field similar to that of the resistive electromagnet. On the other hand, the latter device is believed to be too complex in shape and too difficult to assemble for it to be a desirable system.
Although the permanent magnet device has been believed a truly ideal system for NMR-CT application, there have not been any actual devices which can provide the advantages of a permanent magnet due to the lack of enough strong material and sufficiently advanced design technology.